It allows you to check the look of your post without actually committing it to the board.
D'oh! I'm a complete loser. The button was just staring me in the face and I never thought to use it...Thanks for bringing it to my attention, Ice Cat. Have I told you lately that you rock?
---Human beings, who are almost unique in having the ability to learn from the experience of others, are also remarkable for their apparent disinclination to do so.
Douglas Adams, "Last Chance to See"